The present invention relates to a low loss corrective network and, more particularly, to an equalization circuit arrangement which is employed in transmission systems in order to obtain a desired overall frequency response.
In the prior art, it is generally known to provide corrective networks for improving the frequency-response characteristics of a transmission system. These corrective networks have adjustable components which either increase or decrease the response of a transmission system at a desired frequency or band of frequencies across the overall frequency spectrum. These adjustable components are operative to generate resonances having peaks and valleys so as to set the amplitude (gain or loss) and/or the width of the resonances and/or the center frequency at which the resonance occurs. By using any or all of these parameters, it is possible to either boost or suppress respective portions of the frequency-response characteristic.
One prior-art device employs a variable capacitor, such as a differential capacitor having two similar sets of stator plates and one set of rotor plates. When the rotor is turned, the capacitance of one section is increased, while the capacitance of the other section is decreased. In addition, variable inductors are connected in series with the set of rotor plates. In this manner, resonances may be generated by adjusting either or both the differential capacitor or the variable inductor of each stage.
However, this prior-art device is possessed of many disadvantages. First of all, each stage has its own voltage divider constituted of a pair of resistors having two end terminals connected with the input signal source, and also having a tap point located intermediate the end terminals of the divider which is connected to the rotor plate terminal. The device further includes a separate output resistor, across which the output signal is taken off, and additional resistors connected intermediate each stage and the output resistor.
The effect of this plurality of resistors is to greatly increase the attenuation characteristic of each stage. The cumulative loss of combining several of such stages is, of course, greatly increased as a direct function of the number of such resonant stages that are actually required in a particular application.
Still another drawback of the prior-art device is that its input impedance has a predominantly capacitive nature. This is especially undesirable when it is desired to match the stages to a source having an internal impedance which is entirely resistive.
Finally, the prior-art decoupling arrangements have not proven altogether satisfactory. Thus, undesired resonances, in contrast to desired resonances, have not been reliably prevented from appearing in the output signal.